Blown input diodes on Uni-T UT622E LCR meter

[nekelly]

Hello all, first post here 🙂

This evening I accidentally connected 12v to the input of my month-old (🥴) LCR meter and I heard a pop. I opened it up and found 2 blown diodes. I can replace the M7 ok but I've no idea what the orange/black diode is as it appears to have no markings. Any thoughts? Simple diode or zener maybe? Pic attached.

Thanks, Noel.

[coromonadalix]

maybe 1N4148  ??

Do you have an dmm, and a diode test function

check the one on the photo who seems physically ok  you should have  0.6v  in a direction  and   way higher or infinite in the other way,  but the circuit could play a trick

[nekelly]

Yes, I checked the good one with a DMM and it read 0.56 in the forward direction so I replaced it with a similar one from a laptop board. Hopefully it's ok now!
As an additional check, I put a resistor in series with the good diode and reverse biased it to see if it acted like a zener and there was no breakdown up to 30v.

What originally happened was I touched the test leads to a 12v battery thinking the leads were connected to my DMM but they were attached to my LCR meter. Big whoops!

[Kleinstein]

With 2 back to back diodes it would not make much sense to use zener diodes.  With only 0.56 V this looks like a stronger (higher current) diode than a 1N4148. Chances are the diodes are for protection and there could be additional, not so abvious damage. With a weaker diode the protection would be weaker.

[gamalot]

It's hard to tell, but at least the glass case looks like a 1N4148, if it's not a zener.

We all know that the forward voltage drop of the diode will change with the current, and the figure I found in Vishay's datasheet and the results I measured with my multimeters all prove that the test current change from 0.5mA to 1.5mA will have a great impact on the forward voltage drop of 1N4148.